1. Field of the Invention
The present invention generally relates to an optical switching apparatus, and more particularly to an optical switching apparatus profitable for application to, for example, information processors and picture processors.
2. Description of the Prior Art
FIG. 31 is a drawing showing a conventional optical switch depicted in, for example, "Applied Physics Letters", vol. 45, p.13 (1984), etc. In FIG. 31, reference numeral 1 denotes a p-i-n type device formed by interposing an intrinsic semiconductor region 5, having a semiconductor quantum well layer 4, between a p type semiconductor layer 2 and an n type semiconductor layer 3. Numeral 6 denotes an external electric power supply imposing a predetermined inverse voltage V to the optical bistable device 1. The negative terminal of a direct current power source 7 in the power supply 6 is connected to the p type semiconductor layer 2 of the optical bistable device 1, and the positive terminal of it is connected to the n type semiconductor layer 3 through an electric resistor 8 having the resistance value R. Numeral 9 denotes a light source making an incident light Pin having a predetermined wave length incidence from the side of the p type semiconductor layer 2 of the optical bistable device 1. Numeral 10 denotes a light-receiving device receiving transmission light having been transmitted through the n type semiconductor 3 of the optical bistable device 1. FIG. 32 is a drawing showing a characteristic of the transmission light Pout when the incident light Pin whose intensity is varied is supplied to the optical bistable device 1, that is, a bistable characteristic of the optical bistable device 1. In FIG. 32, numeral 11 denotes an optical bistable region where the transmission light Pout having been transmitted through the optical bistable device 1 can take two transmission states, namely, a high transmission state a and a low transmission state b.
Next, the operation of the conventional optical switching apparatus will be described. The conventional optical switching device is constructed as described above, and a photoelectric current I does not flow in the state where the light is not incident, so the inverse bias voltage V is impressed on the device 1 almost as it is. Now, the semiconductor quantum well layer 4 provided in the device 1 has a sharp absorption peak because of the excitor absorption corresponding to transitions between quantum levels. And, this absorption peak wavelength shifts to the long wavelength side by impressing the inverse bias voltage on the device.
When an incident light Pin having a shorter wavelength than the excitor absorption peak wavelength is supplied to the device 1 from the light source 9, the incident light Pin is transmitted through the device 1 to the side of the n-type semiconductor layer 3 in a high transmitting state (for example, the "a " state of FIG. 32) up to a certain level of incident light intensity (for example, up to the light intensity Pb of FIG. 32), since the amount of light absorbed in the semiconductor quantum well layer 4 is small. Hereby, the light-receiving device 10 receives the light Pout transmitted in a high transmitting state. Hereinafter, such a state, that is to say, the high transmitting state in the optical bistable region, is defined as an ON state.
Next, the positive feedback explained below in detail is generated by further increasing the incident light Pin (the "c" point of FIG. 32). The incident light Pin is absorbed in the semiconductor quantum well layer 4, and the photoelectric current I flows, the amount of which is almost in proportion to the absorption amount. Thereupon, a voltage drop is generated at the electric resistor 8 by the photoelectric current I, then the inverse bias voltage impressed on the device 1 decreases from V to V-IR. The excitor absorption peak wavelength of the semiconductor quantum well layer 4 shifts to the shorter wavelength side and approaches to the wavelength of the incident light Pin by the decrease of the inverse bias voltage impressed on the device 1. Therefore, the light amount absorbed in the semiconductor quantum well layer 4 increases, and the photoelectric current I flowing in the device 1 also increases according to it. Thereupon, the inverse bias voltage impressed on the device 1 decreases the more, then the excitor absorption peak wavelength in the semiconductor quantum well layer 4 shifts to the shorter wavelength side and approaches closer to the wavelength of the incident light Pin. Owing to such the feedback, the light absorption amount absorbed in the semiconductor quantum well layer 4 rapidly increases, and the intensity of the light Pout transmitted through the device 1 rapidly decreases as shown in the "c" point of FIG. 32. The low transmitting state is kept even if the intensity of the incident light Pin from the light source 9 is returned to the light intensity Pb, and the transmitting state moves to the "b" point. Then the light-receiving device 10 receives the light Pout transmitted in the low transmitting state. Hereinafter, such state, that is to say, the low transmitting state in the optical bistable region is defined as an OFF state.
Next, when the incident light intensity of the light Pin is decreased smaller than the bistable region 11 by decreasing the output of the light source 9, the inverse processes to those mentioned above are generated, and the device 1 transfers from the low transmitting state to the high transmitting state. As a result, the light Pout transmitted in the high transmitting state of light is detected by the light-receiving device 10, and the device 1 turns to the ON state, by returning the intensity of the incident light Pin from the light source 9 to the device 1 to the light intensity Pb.
Because the transmission light Pout of the optical bistable device 1 takes two transmission states a and b to a certain incident light Pin's intensity as explained above, the optical bistable device 1 is greatly expected to be used as optical memories of optical information processors, optical logic circuits, and so forth, by utilizing the characteristic capable of switching the two states freely.
Because the conventional optical switching apparatus has the construction above described, it cannot perform the reversible switching of the two light transmission states only by imposing purely optical pulses to the apparatus, and this makes it difficult to control the light source 9 on the occasion of using it in such high speed circuits as optical information processors. Namely, in case of changing the optical bistable device 1 from the high transmission state a to the low transmission state b, it is needed to make the device 1 be in the state of the bistable region 11 by imposing the bias light Pb, and further make the device 1 be in the off state by superimposing the positive trigger pulse light Pb exceeding the bistable region 11. However, a negative pulse light making the optical bistable device 1 be in the state under the optical bistable state 11 is needed in case of changing it from the low transmission state b to the high transmission state a. Hereby, the conventional optical switching apparatus cannot attain whole light type switches using only purely optical pulses.